MHW did 3.5 -4 million.
Considering the series's past, with neither 3DS or SW entries showing this breakout factor, I expect something to the series usual performance: 3.5 - 5 million.
What SW entries? A port of an expansion?
Various franchises’ best selling entries before and after Switch.
Zelda: 1.65m (Zelda 1; NES) -> 2m+ (BOTW)
3D Mario: 2.15m (3D Land; 3DS) -> 2.46m+ (Odyssey)
Smash Bros: 2.6m (Brawl; Wii) -> 4.64m+ (Ultimate)
Animal Crossing: 5.5m (New Lead; 3DS) -> 7.2m+, best selling game of all time in Japan (New Horizons)
Barring any bad outliers (Bad PR, Abysmal reviews, etc.) I fail to see how the same won’t be true for Monster Hunter.